Hydrogeology and Simulation
of Ground-Water Flow and Land-Surface Subsidence in the Chicot and Evangeline
Aquifers, Houston Area, Texas

By Mark C. Kasmarek and Eric W. Strom

U.S. Geological Survey
Water-Resources Investigations Report 02–4022

Document Accessibility: Adobe Systems Incorporated has information about PDFs
and the visually impaired. This information provides tools to help make PDF
files accessible. These tools convert Adobe PDF documents into HTML or ASCII
text, which then can be read by a number of common screen-reading programs that
synthesize text as audible speech. In addition, an accessible version of Acrobat
Reader 5.0 for Windows (English only), which contains support for screen readers,
is available. These tools and the accessible reader may be obtained free from
Adobe at http://access.adobe.com/.

Graph showing sensitivity of the model of the Chicot and Evangeline
aquifers, Houston area, Texas, to changes in aquifer properties and ground-water
withdrawal

42.

Graph showing sensitivity of the model of the Chicot and Evangeline
aquifers, Houston area, Texas, to changes in clay and sand storage properties

Tables

1.

Stress periods used in the model of the Chicot and Evangeline
aquifers, Houston area, Texas

2.

Root-mean-square errors of simulated water levels in the Chicot and Evangeline
aquifers, Houston area, Texas, 1977 and 1996

Abstract

In November 1997, the U.S. Geological Survey, in cooperation with the City
of Houston Utilities Planning Section and the City of Houston Department of
Public Works & Engineering, began an investigation of the Chicot and Evangeline
aquifers in the greater Houston area in Texas to better understand the hydrology,
flow, and associated land-surface subsidence. The principal part of the investigation
was a numerical finite-difference model (MODFLOW) developed to simulate ground-water
flow and land-surface subsidence in an 18,100-square-mile area encompassing
greater Houston.

The focus of the study was Harris and Galveston Counties, but other counties
were included to achieve the appropriate boundary conditions. The model was
vertically discretized into three 103-row by 109-column layers resulting in
a total of 33,681 grid cells. Layer 1 represents the water table using a specified
head, layer 2 represents the Chicot aquifer, and layer 3 represents the Evangeline
aquifer.

Simulations were made under transient conditions for 31 ground-water-withdrawal
(stress) periods spanning 1891–1996. The years 1977 and 1996 were chosen
as potentiometric-surface calibration periods for the model. Simulated and measured
potentiometric surfaces of the Chicot and Evangeline aquifers for 1977 match
closely. Water-level measurements indicate that by 1977, large ground-water
withdrawals in east-central and southeastern areas of Harris County had caused
the potentiometric surfaces to decline as much as 250 feet below sea level in
the Chicot aquifer and as much as 350 feet below sea level in the Evangeline
aquifer. Simulated and measured potentiometric surfaces of the Chicot and Evangeline
aquifers for 1996 also match closely. The large potentiometric-surface decline
in 1977 in the southeastern Houston area showed significant recovery by 1996.
The 1996 centers of potentiometric-surface decline are located much farther
northwest. Potentiometric-surface declines of more than 200 feet below sea level
in the Chicot aquifer and more than 350 feet below sea level in the Evangeline
aquifer were measured in observation wells and simulated in the flow model.

Simulation of land-surface subsidence and water released from storage in the
clay layers was accomplished using the Interbed-Storage Package of the MODFLOW
model. Land-surface subsidence was calibrated by comparing simulated long-term
(1891–1995) and short-term (1978–95) land-surface subsidence with
published maps of land-surface subsidence for about the same period until acceptable
matches were achieved.

Simulated 1996 Chicot aquifer flow rates indicate that a net flow of 562.5
cubic feet per second enters the Chicot aquifer in the outcrop area, and a net
flow of 459.5 cubic feet per second passes through the Chicot aquifer into the
Evangeline aquifer. The remaining 103.0 cubic feet per second of flow is withdrawn
as pumpage, with a shortfall of about 84.9 cubic feet per second supplied to
the wells from storage in sands and clays. Water simulated from storage in clays
in the Chicot aquifer is about 19 percent of the total water withdrawn from
the aquifer.

Simulated 1996 Evangeline aquifer flow rates indicate that a net flow of 14.8
cubic feet per second enters the Evangeline aquifer in the outcrop area, and
a net flow of 459.5 cubic feet per second passes through the Chicot aquifer
into the Evangeline aquifer for a total inflow of 474.3 cubic feet per second.
A greater amount, 528.6 cubic feet per second, is withdrawn by wells; the shortfall
of about 54.8 cubic feet per second is supplied from storage in sands and clays.
Water simulated from storage in clays in the Evangeline aquifer is about 10
percent of the total water withdrawn from the aquifer.